1. Field of the Invention
The present invention relates to a radio communication apparatus and an impedance matching method and more particularly, to a radio communication apparatus with a retractable antenna and an impedance matching method of a radio communication apparatus of this sort, which make it possible to match the impedance between the antenna and the circuit connected to the antenna in a situation where the antenna environment is similar to a free space and another situation where the antenna environment is dissimilar from a free space.
2. Description of the Related Art
The portable telephone, which is a typical one of radio communication apparatuses, has several operation states, such as the stand-by state for waiting incoming messages or data, the dial-inputting state for inputting phone numbers, and the communication state for the transmission or reception operation. These operation states can be categorized into two classes, i.e., the xe2x80x9ccommunication statesxe2x80x9d where the telephone performs the transmission or reception operation and the xe2x80x9cnon-communicationxe2x80x9d states where the telephone does not perform the transmission or reception operation.
In the xe2x80x9cnon-communication statesxe2x80x9d, the environment of the antenna is similar to a free space and therefore, it can be approximated as a free space. In the xe2x80x9ccommunication statesxe2x80x9d, however, the environment of the antenna is not similar to a free space. This is because the telephone is often used in a location near the head of the user for the purpose of exchanging voice messages using the microphone and receiver on the telephone. Thus, it is typical that the environment of the antenna is unable to be approximated as a free space in the communication states.
Moreover, considering the state that the telephone is used for data transmission, this state belongs to the xe2x80x9ccommunication statesxe2x80x9d; in this state, however, the telephone is usually used in a location far from the head of the user. This is because the microphone and receiver for exchanging voice messages is not necessary and because the telephone is usually operated along with a portable computer. As a result, the environment of the antenna can be approximated as a free space in the data transmission state.
Accordingly, if the operation states of the telephone are classified with respect to the environment of the antenna, they are categorized into two situations, i.e., the first situation similar to a free space and the second situation dissimilar from a free space. Thus, the input impedance of the antenna needs to be changed according to which one of these two situations is applicable.
Additionally, the antenna of the portable telephone is usually retractable and therefore, the environment of the antenna varies according to whether or not the antenna is retracted into the casing too.
Taking the above-described change in the antenna environment into consideration, conventionally, the configuration of portable telephones has been designed and developed. This is applicable to any other radio communication apparatuses having a retractable antenna.
FIG. 1 shows schematically the configuration of a prior-art radio communication apparatus, which is configured as a portable telephone.
As shown in FIG. 1, the prior-art radio communication apparatus 101 comprises a whip antenna 102, a matching circuit 105, a switch 106 with three terminals for switching the transmission and reception operations, a terminal matching circuit 107, a transmission circuit 111, a reception circuit 112, a control circuit 113, and a casing 120. The matching circuit 105, the switch 106, the terminal matching circuit 107, the transmission circuit 111, the reception circuit 112, and the control circuit 113 are located in the casing 120. The antenna 102 is fixed to the casing 120 so as to be retractable into the casing 120.
The whip antenna 102 includes a straight element 104 and a helical element 103. The straight element 104 is supported by the casing 120 so as to be extendable from the casing 120 to the outside and retractable into its inside. The helical element 103 is connected in series to the straight element 104. The antenna 102 emits a transmitting signal TS supplied from the transmission circuit 111 in the form of a radio wave and generates a reception signal RS from a radio wave received from the outside.
In the state where the straight element 104 is retracted into the casing 120 (which may be said that the whip antenna 102 is retracted hereinafter), only the helical element 103 protrudes from the casing 120. On the other hand, in the state where the straight element 104 is extended to the outside of the casing 120 (which may be said that the whip antenna 102 is extended hereinafter), both the helical and straight elements 103 and 104 protrude from the casing 120.
One terminal of the terminal matching circuit 107 is connected to a connector 121 and the other terminal thereof is connected to the ground. The circuit 107 has predetermined, specific impedance.
One terminal of the matching circuit 105 is connected to a connector 122 and the other terminal thereof is connected to a first terminal of the switch 106. The circuit 105 serves to match the impedance between the antenna 102 and the transmission or reception circuit 111 or 112.
When the whip antenna 102 is retracted into the casing 120, as shown in FIG. 1, the bottom end of the straight element 104 is contacted with the connector 121. Thus, the terminal matching circuit 107 is connected to the bottom end of the element 104. The upper part of the element 104 is contacted with the connector 122, thereby connecting the matching circuit 105 to the element 104.
When the whip antenna 102 is extending from the casing 120, the bottom end of the straight element 104 is apart from the connector 121 and as a result, the terminal matching circuit 107 is disconnected from the element 104. The element 104 is kept in contact with the connector 122 and as a result, the matching circuit 105 is kept connected to the element 104 in this state.
The second and third terminals of the switch 106 are connected to the output terminal of the transmission circuit 111 and the input terminal of the reception circuit 112, respectively. The switch 106 alternately connects one of the transmission and reception circuits 111 and 112 to the matching circuit 105.
The transmission circuit 111 generates the transmission signal TS by modulating the carrier wave with specific transmission data. The data TS is outputted from the output terminal of the circuit 111 to the whip antenna 102 by way of the switch 106 and the matching circuit 105. The operation of the circuit 111 is controlled by a transmission control signal TCS supplied from the control circuit 113.
The reception signal RS, which is generated by the antenna 102, is inputted into the input terminal of the reception circuit 112 by way of the matching circuit 105 and the switch 106. The reception circuit 112 demodulates the reception signal RS thus supplied and extracts the data contained in the signal RS. The operation of the circuit 112 is controlled by the reception control signal RS.
If the reception signal RS contains a disconnection signal DCS, the reception circuit 112 outputs the signal DCS to the control circuit 113. In this case, the circuit 113 stops the reception operation of the circuit 112 using the control signal RCS.
The control circuit 113 generates the switch control signal SCS, the transmission control signal TCS, and the reception control signal RCS and then, supplied them to the switch 106, the transmission circuit 111, and the reception circuit 112, respectively.
The prior-art radio communication apparatus 101 operates in the following way.
On signal transmission, the switch 106 is operated to connect the transmission circuit 111 to the matching circuit 105. The transmission signal TS outputted from the transmission circuit 111 is supplied to the whip antenna 102 through the matching circuit 105 and the switch 106. The signal TS thus supplied is emitted to the outside or the air in the form of radio wave.
On signal reception, the switch 106 is operated to connect the reception circuit 112 to the matching circuit 105. The reception signal RS supplied from the antenna 102 is inputted into the reception circuit 112 through the matching circuit 105 and the switch 106. The signal RS thus inputted is demodulated to extract the data contained therein.
Thus, according to the switch control signal SCS from the control circuit 113, the electrical connection to the antenna 102 is switched to the transmission or reception circuit 111 or 112.
As explained above, when the whip antenna 102 is extended, both the helical and straight elements 103 and 104 of the whip antenna 102 protrude from the casing 120. As a result, both the elements 103 and 104 provide the specific antenna function. In this state, the terminal matching circuit 107 is inactive and therefore, only the matching circuit 105 provides the impedance matching function between the antenna 102 and the reception or transmission circuit 111 or 112.
On the other hand, when the whip antenna 102 is retracted, only the helical element 103 protrudes from the casing 120. As a result, only the element 103 provides the specific antenna function. In this state, the terminal matching circuit 107 is active and compensates the input impedance of the antenna 102. In other words, when only the helical element 103 is available, the input impedance of the antenna 102 is compensated so as to match the impedance between the antenna 102 and the reception or transmission circuit 111 or 112. Thus, like the state where the whip antenna 102 is extended, the impedance matching operation between the antenna 102 and the reception or transmission circuit 111 or 112 is accomplished by the matching circuit 105.
Usually, the matching circuit 105 is adjusted in such as way that the impedance is optimized or matched between the antenna 102 and the reception or transmission circuit 111 or 112 when the whip antenna 102 is extended. This adjustment for desired impedance matching is accomplished under the supposition that the antenna 2 is placed in a free space (i.e., in the first situation).
In the state where the apparatus 101 is located in the vicinity of the head of the user (i.e., in the second situation), however, the input impedance of the helical element 103 varies due to the effect of the head. Thus, the impedance matching condition will deviate. The deviation in impedance matching condition when the antenna 102 is retracted is larger than that when the antenna 102 is extended.
FIG. 2 is a Smith chart indicating the input impedance of the whip antenna 102 (substantially, the helical element 103) in the second situation where the apparatus 101 is located near the head of the user. As seen from FIG. 2, the curve a1 showing the input impedance of the antenna 102 is shifted below from the center line b1, which means that the input impedance characteristic has degraded.
FIG. 3 is a graph showing the return loss characteristic between the antenna 102 (substantially, the helical element 103) and the transmission or reception circuit 111 or 112 in the second situation. As seen from FIG. 3, the return loss is greater than the reference value of xe2x88x925 dB for transmission and reception within the frequency range W1. This means that the return loss is excessively large.
If the impedance matching is determined to appear in the second situation where the environment of the antenna 102 is dissimilar from a free space, the above-described characteristic degradation can be avoided. In this case, however, there arise a problem that the input impedance is not optimized in the first situation where the environment of the antenna 102 is similar to a free space.
Accordingly, when the whip antenna 102 is retracted into the casing 120, it is difficult to realize desired impedance matching in both the first and second situations. This means that the impedance matching is not accomplished in any one of these two situations.
Recently, there has been the strong trend to further miniaturize portable radio communication apparatuses such as portable telephones and as a result, the distance of the helical element 103 from the head of the user has been decreasing. Thus, the impedance characteristic of the antenna 102 in the above-described first and second situations tends to vary or fluctuate in a wider range and the return loss tends to be more conspicuous.
Accordingly, an object of the present invention is to provide a radio communication apparatus and an impedance matching method thereof that realize a desired impedance matching state between a retractable antenna and a circuit connected thereto in both a situation where the antenna environment is dissimilar from a free space.
Another object of the present invention is to provide a radio communication apparatus and an impedance matching method thereof that realize a desired impedance matching state between a retractable antenna and a circuit connected thereto even when the antenna is retracted.
Still another object of the present invention is to provide a radio communication apparatus and an impedance matching method thereof that that decrease the return loss of a retractable antenna in each of a situation where the antenna environment is similar to a free space and a situation where the antenna environment is dissimilar from a free space.
The above objects together with others not specifically mentioned will become clear to those skilled in the art from the following description.
According to a first aspect of the present invention, a radio communication apparatus is provided, which comprises:
(a) a casing;
(b) an antenna retractable into the casing;
the antenna having a first element and a second element joined together;
the first element being placed outside even when the antenna is retracted;
the second element being retracted into the casing when the antenna is retracted;
the second element being connected to an internal circuit provided in the casing;
(c) a terminal matching circuit for matching an impedance of the antenna to that of the internal circuit in a first situation where an environment of the antenna is similar to a free space and a second situation where an environment of the antenna is dissimilar from a free space;
the terminal matching circuit being configured to provide at least two terminal impedance values for the antenna; and
(d) a control circuit for controlling an operation of the terminal matching circuit corresponding to whether the apparatus is placed in the first situation or the second situation;
the control circuit controlling the terminal matching circuit in such a way that a first one of the at least two terminal impedance values is selected when the apparatus is placed in the first situation and a second one of the at least two terminal impedance values is selected when the apparatus is placed in the second situation.
With the radio communication apparatus according to the first aspect of the invention, the terminal matching circuit is provided for matching the impedance of the retractable antenna to the internal circuit in the first situation where an environment of the antenna is similar to a free space and the second situation where an environment of the antenna is dissimilar from a free space. The terminal matching circuit is configured to provide at least two terminal impedance values for the circuit.
Moreover, the control circuit is provided for controlling the operation of the terminal matching circuit corresponding to whether the apparatus is placed in the first situation or the second situation. The control circuit controls the terminal matching circuit in such a way that the first one of the at least two terminal impedance values is selected when the apparatus is placed in the first situation and the second one of the at least two terminal impedance values is selected when the apparatus is placed in the second situation.
As a result, a desired impedance matching state between the retractable antenna and the inner circuit connected thereto can be realized in each of the first situation where the antenna environment is similar to a free space and the second situation where the antenna environment is dissimilar from a free space.
Because of the impedance matching, the return loss of the antenna can be decreased.
If the at least two terminal impedance values for the antenna are properly set to meet the requirement when the antenna is retracted into the casing or not, a desired impedance matching state between the retractable antenna and the inner circuit can be realized in each of the first and second situations independent of whether the antenna is retracted or not.
In a preferred embodiment of the apparatus according to the first aspect of the invention, the terminal matching circuit comprises a first termination subcircuit, a second termination subcircuit, and a switch for switching the first and second termination subcircuit by a control signal from the control circuit.
In this embodiment, preferably, the first termination subcircuit is connected to the second element of the antenna when the apparatus is in the first situation, and the second termination subcircuit is connected to the second element of the antenna when the apparatus is in the second situation.
Alternately, the first termination subcircuit is connected to the second element of the antenna when the apparatus is in the first situation, and the first and second termination subcircuits are connected to the second element of the antenna when the apparatus is in the second situation.
In another preferred embodiment of the apparatus according to the first aspect of the invention, the first situation includes a stand-by state and a data communication state of the apparatus and the second situation includes a voice message exchange state.
In still another preferred embodiment of the apparatus according to the first aspect of the invention, an earphone detection circuit outputting an earphone detection signal to the control circuit is additionally provided. The control circuit controls the terminal matching circuit responsive to the earphone detection signal in such a way that the first one of the at least two terminal impedance values is selected.
In a further preferred embodiment of the apparatus according to the first aspect of the invention, the control circuit is designed to receives an operation selection signal. The control circuit controls the terminal matching circuit responsive to the operation selection signal in such a way that the first one of the at least two terminal impedance values is selected when the apparatus is placed in the first situation and the second one of the at least two terminal impedance values is selected when the apparatus is placed in the second situation.
In this embodiment, it is preferred that the first one of the at least two terminal impedance values is selected when the operation selection signal is a signal ending voice message exchange or a disconnection signal sent from a remote apparatus, and the second one of the at least two terminal impedance values is selected when the operation selection signal is a signal starting voice message exchange.
As the operation selection signal, a signal indicating an operation state of the apparatus generated from the reception data may be used.
According to a second aspect of the present invention, an impedance matching method of a radio communication apparatus is provided. The apparatus comprises
(i) a casing;
(ii) an antenna retractable into the casing;
the antenna having a first element and a second element joined together;
the first element being placed outside even when the antenna is retracted;
the second element being retracted into the casing when the antenna is retracted;
the second element being connected to an internal circuit provided in the casing;
(iii) a terminal matching circuit for matching an impedance of the antenna to that of the internal circuit; and
(iv) a control circuit for controlling an operation of the terminal matching circuit.
The method according to the second aspect comprises the steps of:
(a) providing at least two terminal impedance values for the antenna to the terminal matching circuit so as to match the impedance of the antenna to that of the internal circuit in a first situation where an environment of the antenna is similar to a free space and a second situation where an environment of the antenna is dissimilar from a free space; and
(b) operating the control circuit in such a way that a first one of the at least two terminal impedance values is selected when the apparatus is placed in the first situation and a second one of the at least two terminal impedance values is selected when the apparatus is placed in the second situation.
With the impedance matching method of a radio communication apparatus according to the second aspect of the invention, because of substantially the same reason, a desired impedance matching state between the retractable antenna and the inner circuit connected thereto can be realized in each of the first and second situations. Because of the impedance matching, the return loss of the antenna can be decreased.
If the at least two terminal impedance values for the antenna are properly set to meet the requirement when the antenna is retracted into the casing or not, a desired impedance matching state between the retractable antenna and the inner circuit can be realized in each of the first and second situations independent of whether the antenna is retracted or not.
In a preferred embodiment of the method according to the second aspect of the invention, the terminal matching circuit is configured to comprise a first termination subcircuit, a second termination subcircuit, and a switch. A control signal is sent from the control circuit to switch the first and second termination subcircuits.
In this embodiment, preferably, the first termination subcircuit is connected to the second element of the antenna when the apparatus is in the first situation, and the second termination subcircuit is connected to the second element of the antenna when the apparatus is in the second situation.
Alternately, the first termination subcircuit is connected to the second element of the antenna when the apparatus is in the first situation, and the fist and second termination subcircuits are connected to the second element of the antenna when the apparatus is in the second situation.
In another preferred embodiment of the method according to the second aspect of the invention, the first situation is set to include a stand-by state and a data communication state of the apparatus and the second situation is set to include a voice message exchange state.
In still another preferred embodiment of the method according to the second aspect of the invention, an earphone detection circuit outputting an earphone detection signal to the control circuit is additionally provided. The control circuit controls the terminal matching circuit responsive to the earphone detection signal in such a way that the first one of the at least two terminal impedance values is selected.
In a further preferred embodiment of the method according to the second aspect of the invention, the control circuit receives an operation selection signal. The control circuit controls the terminal matching circuit responsive to the operation selection signal in such a way that the first one of the at least two terminal impedance values is selected when the apparatus is placed in the first situation and the second one of the at least two terminal impedance values is selected when the apparatus is placed in the second situation.
In this embodiment, it is preferred that the first one of the at least two terminal impedance values is selected when the operation selection signal is a signal ending voice message exchange or a disconnection signal sent from a remote apparatus, and the second one of the at least two terminal impedance values is selected when the operation selection signal is a signal starting voice message exchange.
As the operation selection signal, a signal indicating an operation state of the apparatus generated from the reception data may be used.